A vehicle-mounted device known in the related art, which is capable of detecting an obstacle based upon a photographic image captured with a camera, extracts an image area in the photographic image that remains unchanged over time and suspends obstacle detection based upon a ratio of the image area to the entire photographic image area (see PTL1). PTL1 teaches that obstacle detection is resumed once the ratio of the image area remaining unchanged over time to the entire photographic image area, becomes lower.